Problem: Evaluate $-\dfrac{2}{5}-x+\left(-\dfrac{9}{10}\right)$ where $x = \dfrac{4}{5}$.
Explanation: Let's substitute ${x = \dfrac{4}{5}}$ into the expression. $\begin{aligned} &\phantom{=} -\dfrac{2}{5}-{x}+\left(-\dfrac{9}{10}\right) \\\\ &= -\dfrac{2}{5}-{\dfrac{4}{5}}+\left(-\dfrac{9}{10}\right) \end{aligned}$ $\begin{aligned} &= -\dfrac{2}{5}-\dfrac{4}{5}-\dfrac{9}{10} \\\\ &=-\dfrac{6}{5}-\dfrac{9}{10} \end{aligned}$ $=-\dfrac{21}{10}$